Her promotion makes her one of the most senior women within WPP, alongside the group chief counsel, Andrea Harris, and the global chief executive of J Walter Thompson, Tamara Ingram, and it signals the importance of Group M within the world’s biggest advertising group.

WPP has also merged other agencies, including putting Possible and Salmon into Wunderman and Neo@Ogilvy into Mindshare, in recent months as part of a move to simplify the group and cut costs.

WPP owns "more than 160 companies," according to the annual report, although it maintains it already has a relatively simple structure because it has a dozen or so key operating companies.

"Lindsay will oversee our 50 global client teams and help to deliver on our core strategic priority of horizontality," said Sorrell, in a memo to staff. "Our single-most-important task is to ensure we provide our clients with a seamlessly integrated offer, as they demand greater simplicity and efficiency from their marketing services partners."

Pattison, 44, represents WPP as a director on the board of Chime, the private-equity-backed owner of VCCP, in which WPP has a minority stake.

She also launched Walk the Talk, an initiative to help senior women to advance in their careers, at Maxus. That program is being adopted across WPP.

WPP’s share price has fallen from £19 in March to close to £13, about $17, as sales have gone into reverse, a decline Sorrell has blamed partly on FMCGs cutting spend.